Resistance is resistance, whether it's your tester or the sender. Current, voltage, resistance - nothing more. How much does the voltage drop at the sender when you connect the gauge wire? How much does the voltage drop when you connect your tester and the gauge works? Should be the same, if the resistance and the path to ground is the same. Has to be your ground from the sender to the block. Or you are comparing performance through the Jeep wiring to performance on the bench. Where are you grounding your tester? Try putting a clip lead from the sender body to somewhere else on the engine that's a good ground. What do you mean, "outside?" With the cluster out of the dash?